Sulfur dioxide and nitric oxide are two of the major contributors to air pollution in many of the industrial areas of the world today. A variety of approaches have been taken to reduce power plant emission levels of sulfur dioxide and nitric oxide. For a summary of the processes considered commercially feasible to remove sulfur dioxide from a flue gas see CEP Vol. 71, No. 5, pgs. 55-71.
In the direct treatment of the flue gas by catalytic reduction, the major problems encountered are the formation of hydrogen sulfide with water-containing streams and the reformation of sulfur dioxide with oxygen-containing streams. In particular, with the water-containing streams, it is generally found that hydrogen sulfide formation occurs with all catalysts presently available, including copper on alumina.
With oxygen levels greater than 0.5%, there is a noticeable decline in sulfur dioxide removal when sulfur dioxide is reduced with carbon monoxide. Further, the carbonyl sulfide which is present in the reduction sequence for sulfur dioxide can be reoxidized to sulfur dioxide and carbon dioxide. Oxygen also reacts with hydrogen sulfide as well as carbonyl sulfido and thus sulfur dioxide is reformed and removal efficiency declines whether water and/or oxygen as present in the stream to be treated.
The removal of sulfur dioxide per se from a flue gas stream and its subsequent catalytic reduction to elemental sulfur is well known; see U.S. Pat. No. 3,495,941, Van Helden; U.S. Pat. No. 3,615,231, Cullom; and U.S. Pat. No. 3,764,665, Groenendaal. Methane reduction of sulfur dioxide combined with a two-stage Claus process has been employed by Allied Chemical Company. Sulfur removal before and between the stages of the Claus unit is employed due to thermodynamic limitations and to prevent combinations of the sulfur on the Claus catalyst.
Sequential reduction of sulfur dioxide, including forming carbonyl sulfide in a first stage and reacting sulfur dioxide with carbonyl sulfide in a second stage has been broadly shown; see U.S. Pat. No. 1,710,141, Benner et al; and U.S. Pat. No. 2,213,787, Von Girsewald. These prior art processes generally do not account for the formation of reaction products which then have to be separately treated and/or they do not use a simple and economical manner for processing sulfur dioxide in a flue gas to elemental sulfur. More importantly, the processes do not teach the formation of carbon disulfide. Processes for the manufacture of carbon disulfide are also well known; see U.S. Pat. Nos. 3,568,121; 2,935,380; 2,767,058; and 2,767,059. However, these processes are not directed to the formation of carbon disulfide from sulfur dioxide which has been removed from a flue gas.